NowhereMan:
The monster that started this whole search was BattleTech. What I'm wanting to find is a handful of other games that allow for the following general idea, in this case we'll go with a mech:
Player's mech is fired upon by the enemy.
A roll, based on facing, is made to determine where the mech is hit.
The attack hits the mech's left leg and penetrates the armor, causing damage to the mech's systems.
Another roll is made, determining that the mech's left-side stabilizers were hit, causing the mech to be slower-moving and a chance for it to fall any time it moves.
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In a different scenario, you could replace the mech with a tank, APC, a starship, or a plane.
Battletech does have full systems for all kinds of combat.
Battletech itself covers infantry, powersuits, vehicles (wheeled, tracked, hover, WiGE), aircraft (VTOL, conventional, aerospace), shuttles, dropships, and system craft, and and all the way up to multi-million ton capital ships.
Some of them work better than others.
Detailed vehicular combat up to and including some kind of systems degradation is something I've been looking for as well to layer into an RPG, but it's a pretty rare bird.